In recent years, various polymer films have been used widely as electrical parts or devices, such as capacitor films and flexible printed circuit boards, in the field of electronic and electrical industry.
Flexible printed circuit boards (FPC) with electronic parts mounted thereon, such as IC, are required to have good service properties such as high heat resistance, moisture resistance, flex resistance, nonflammability, copper peeling resistance and the like. Most of these service properties are however dependent on the corresponding properties of the polymer film employed as a base material. The utility of polymer films is expected to expand further to such fields as requiring a high level of heat resistance, for example, insulating films for motors and transformers.
The polymer films used as these electrical parts or devices tend to be used after being soldered. Upon soldering, the temperature of fused solder reaches as high as about 260.degree. C. There are, however, not many polymer films which can withstand such a high temperature processing. On the other hand, those having excellent solder heat resistance are costly and hence involve a practical problem as industrial materials from the economical viewpoint.
Poly(arylene sulfides) which may hereinafter be abbreviated as "PASs", such as poly(phenylene sulfides) which may hereinafter be abbreviated as "PPSs", have been known as resins having excellent heat resistance and chemical resistance, and biaxially-stretched films thereof have also been known conventionally (Japanese Patent Publication No. 5101/1984 and Japanese Patent Publication No. 44968/1984). The level of solder heat resistance required for practical application is such that no changes in external appearance, such as swelling and distortion, be observed when immersed in fused solder at 260.degree. C. for 10 seconds. Under such high-temperature conditions, however, films made of a PAS alone inevitably undergo changes in external appearance so that they can hardly be regarded as films meeting the required level of solder heat resistance.
As heat-resistant resins having a melting point of about 300.degree. C. or higher, polyether ether ketones which may hereinafter be abbreviated as "PEEKs" and polyether ketones which may hereinafter be abbreviated as "PEKs" have recently been developed. They can be easily formed into various shaped products such as films. For example, it has however been considered difficult to manufacture stretched films from a PEEK on an industrial scale because the PEEK contains aromatic rings in its backbone, its molecular chain is stiff and its stretchability is thus poor. In addition, these resins use expensive fluorine-substituted aromatic compounds such as 4,4'-difluorobenzophenone as their raw materials, and hence there exist limitations to the reduction of their costs.
The present inventors proceeded with an investigation to obtain stretched films from a PAS to which a resin having good heat resistance had been added with a view toward providing PAS films of improved solder heat resistance.
As a result, it has been found that a composition comprising a PEEK and a substantially-linear and high-molecular PAS provides a heat-resistant film by biaxial stretching (Japanese Patent Application No. 316306/1987). The composition, however, is not sufficient in the compatibility of both the resins and in addition, it should contain the expensive PEEK as a principal component in order to manufacture such heat resistant films stably by melt extrusion. Therefore, use of this resin is not advantageous from the economical viewpoint.
On the other hand, based on the assumption that poly(arylene thioether-ketones) (hereinafter abbreviated as "PTKs") could become heat-resistant thermoplastic resins like PEEKs and PEKs owing to their similarity in chemical structure, PTKs have been studied to some extent to date.
There are some disclosure on PTKs, for example, in German Offenlegungsschrift DE-3405523Al, Japanese Patent Application Laid-Open No. 58435/1985, European Patent Application Laid-Open No. 0,135,938 Al, U.S. Pat. No. 3,819,582, Indian J. Chem., 21A, 501-502 (May, 1982), Japanese Patent Application Laid-Open No. 221229/1986, U.S. Pat. No. 4,716,212, U.S. Pat. No. 4,690,972, European Patent Application Laid-Open No. 0,270,955 Al, European Patent Application Laid-Open No. 0,274,754 Al, European Patent Application Laid-Open No. 0,285,783 Al, European Patent Application Laid-Open No. 0,280,325 Al and European Patent Application Laid-Open No. 0,287,009 Al.
The PTKs described in the above publications, however, have poor melt stability and upon their melt processing, they may lose crystallinity or may undergo a crosslinking reaction or foaming, resulting in an increase in melt viscosity. It has hence been difficult to carry out melt forming or molding in accordance with a conventional melt processing technique such as injection molding or extrusion.
With the foregoing in view, the present inventors found that PTKs significantly improved in melt stability compared with the conventionally-known PTKs can be obtained by designing a polymerization process and conducting polymerization without any polymerization aid while paying attention to the selection of a charge ratio of monomers, the shortening of the polymerization time at high temperatures, the selection of a material for a polymerization reactor and optionally, by applying a stabilization treatment in a final stage of the polymerization. PTKs thus obtained are melt-stable ones which can be formed or molded by a conventional melt processing technique. (Japanese Patent Application Laid-Open No. 54031/1989).
These melt-stable PTKs alone or as compositions wherein 100 parts by weight of a PTK are blended with up to 100 parts by weight of another miscible thermoplastic resin can give lubricative stretched films having significant heat-resistance and strength (Japanese Patent Application Laid-Open No. 45439/1989). However, these PTKs also use expensive raw materials, leading to a problem that films made of one or more of such PTKs as principal component become inevitably costly.
With a view toward obtaining films having heat resistance superior to the films of PAS alone from a composition wherein a PAS, a principal component, is added with a resin having good heat resistance, the present inventors have proceeded with a further investigation.
It is necessary to add a resin superior in heat resistance to PAS in order to improve the heat resistance of PAS films. However, addition of such a heat resistant resin in a small quantity does not always contribute to the improvement of heat resistance compared with films of PAS alone. The state of mixing of both the components is important for the improvement of heat-resistance even if the resin is added in a small amount. In the state that a heat-resistant resin as a minor component has aggregated and is dispersed as an island component in the composition, the heat resistance of PAS forming a sea component can not be improved. That is to say, it is necessary for the heat-resistant resin as the minor component to form to some extent a continuous layer in the composition. Desired as a heat-resistant resin added to PAS is therefore a resin which has a structure chemically similar to that of PAS, does not undergo thermal decomposition during melt processing, can be mixed sufficiently without extreme difference in viscosity during melt mixing, and can be easily stretched into films.
Under the present situation, however, such a heat-resistant resin has not been obtained easily.